It is known that glass is an amorphous material obtained by melting crystalline compounds followed by cooling down of the melted mass.
On the contrary glass-ceramics are vitreous systems that, when brought to a temperature T1 higher then their glass transition temperature Tg and lower then the melting temperature, present the formation of crystal nuclei (homogeneous or heterogeneous) with following crystal growth.
The known glass-ceramics are normally prepared by melting the starting materials into a fused mass which thereafter is cooled down by quenching giving vitreous amorphous scales, technically called frits, which are thereafter applied, possibly in combination with other additives, on the appropriate porous substrate (or are added to the other necessary compound in a ceramic body if porcelain stonewares are desired) and undergo the firing process during which a crystallisation of the vitreous phase takes place and the final wanted ceramic material is obtained.
Alternatively the melted mass can be cooled down and then submitted to appropriate termic cycles which induce the crystallisation in bulk.
In PCT/EP97/04387 (in the name of the same applicant) it is reported that when frits having a determined composition are ground before the firing process products having particularly advantageous properties are obtained.
A very important property of the vitreous frits is their thermal expansion coefficient (α) since they must follow the expansion of the porous substrate to which they are applied during the firing step. This is especially true in the so called double fast firing, single-firing gres technologies for the productions of glazes and engobes.
The above said property plays a critical role for example in the production of glazed or porcelain stoneware tiles which must be perfectly planar.
Up to now, in order to increase the thermal expansion coefficient of the glass-ceramics, two different approaches have been followed:    a) use of frits having an high content of fluorine;    b) use of frits having an high content of alkaline elements.
However, both the above said approaches present serious drawbacks. In fact the use of frits having an high content of fluorine implies pollution problems which require expensive, although often not satisfactory, technologies to be overcome; on the other side the frits having an high content of alkaline elements known up to now usually do not crystallise giving homogeneous glasses which, when used for example in the production of glazes, give products easily attacked by acids and even solve in aqueous media.
From what above said, it is clear the importance to make available new glass-ceramics having the desired properties and in particular an high thermal expansion coefficient.